This type of sensor has many applications as a pressure sensor in particular, allowing, by detection of modulation of the phase of an optical wave, environmental variations to be detected. One of the fields of application in which this type of detection is particularly of interest is underwater detection.
The underwater detection of soundwaves is of fundamental importance for coastal surveillance and certain military applications (for example detection/identification of submarines and surface vessels, the detection of divers or drones, etc.) and for civil applications (bioacoustic applications, monitoring of underwater seismic activity, detection of noise in the environment, etc.). To this end, fiber optic sensors are widely used both for their sensitivity and their compactness. The most sensitive are based on active (laser) or passive Bragg gratings, coupled to a mechanical transducer that converts the radial pressure of the acoustic wave into elongation of the fiber leading to a change in phase or optical wavelength. Distributed feedback fiber laser (DFB-FL) sensors are capable of detecting picostrains i.e. relative variations of −ΔL/L˜10−12, as described in the article “All-optical acoustic array for underwater surveillance”, R. Bouffaron et al., PROCEEDINGS OF SPIE, volume 8794, paper 8794-36, Fifth European Workshop on Optical Fibre Sensors, Krakow 2013.
However, sensors and systems based on Bragg gratings are intrinsically sensitive to variations in the environment (typically temperature and static pressure variations), this placing large constraints on the design of this type of sensor and/or system.
Furthermore, to detect the associated optical phase modulation, precise interferometric measurements must be carried out, which are often a limitation on practical implementation of such systems, in particular when the detection must be executed in a fluctuating environment. Specifically, in this case, servocontrol mechanisms that are often onerous must be implemented in conventional interferometers to maintain quadrature, i.e. the optimal detection condition that allows the sensitivity of the sensor to be maximized.
To meet the needs of this type of application, requiring extreme sensitivities to the quantity to be measured and a relative insensitivity to environmental interference impacting the reading system, the Applicants have proposed in patent application FR 1302640 to use the high phase-change sensitivity delivered by the adaptive holography obtained by mixing two waves in a liquid-crystal light valve (LCLV), this type of valve being known in the prior art for its capacity to produce holograms by converting the intensity variation of an interference pattern into a variation in the index of the liquid crystal, thus inducing an optical phase variation. Typically, the LCLV comprises a thin layer of liquid crystal LC, typically of a thickness comprised between 10 and 200 μm, this layer being placed between two substrates, one of the substrates comprising a photoconductor PC able to convert the light received at N emission wavelengths λi into charge. For example, the layer of liquid crystals is located between a glass substrate and a substrate made of a photoconductor crystal, for example a crystal of BSO, that is sensitive to blue/green wavelengths, electrodes being deposited on the substrates.
Apart from its sensitivity, the passband of this detecting device allows slow fluctuations to be filtered and a sensor that is insensitive to variations in environmental conditions to be produced. In addition, this detecting device is coupled to a multimode fiber optic sensor, which is more sensitive to elongation than a single-mode fiber, and which therefore allows the sensing portion to be simplified and the transducer to even be made redundant.
Nevertheless, this solution has a drawback insofar as the heart of the interferometer is a liquid-crystal light valve that prevents independent optimization of the writing and reading of the hologram (the latter for example being adapted to the working wavelength) and insertion of a step of processing the signal in the interferometer.
To mitigate the aforementioned drawbacks, the Applicants propose a new fiber optic sensor using digital holography, replacing and thus disassociating the liquid-crystal valve with a video camera and a liquid-crystal spatial light modulator.